1. Field of the Invention
The present invention relates to a method, program, and apparatus for copy protection, and more particularly to a method, program, and apparatus for detecting a copy-prohibited document and prohibiting a reproduction of the detected copy-prohibited document. The present invention also relates to a computer readable storage medium storing the above-mentioned program for detecting a copy-prohibited document and prohibiting a reproduction of the detected copy-prohibited document.
2. Discussion of the Background
With recent improvements in image processing and image forming technologies, it has become possible that a high-tech digital color copying apparatus, for  example, can reproduce a monetary document even with a high fidelity such that the reproduced image is hardly distinguished from the original image. Such monetary document is a paper currency, a security, and the like, for example, and will not normally be placed as an object of copying since an authorized currency creation is illegal. Therefore, the high-tech digital color copying apparatus is needed to be provided with a feature of prohibiting a reproduction of a copy-prohibited document such as a monetary document, a confidential document, etc. With this feature, a copy-prohibited document may entirely be prohibited from being reproduced or can be reproduced into a nonreadable image, for example.
In offices, there are many confidential documents which are not necessarily monetary documents but are prohibited from being copied from a viewpoint of trade secrets. These confidential documents are also needed to be prohibited from being copied with the high-tech digital color copying apparatuses.
Under these circumstances, various inventions associated with the above-mentioned high-tech digital color copying apparatus have been conducted to attempt to restrict the capability of an image reproduction with a high fidelity.
Japanese Laid-Open Unexamined Patent Application Publication, No. 06-125459 describes a technique for recognizing a special document such as currency, a security, and so on by comparing input image data with a prestored specific mark by pattern matching and judging that the input image data is a special document when the input image data is recognized as matching the prestored specific mark. Japanese Laid-Open Unexamined Patent Application Publication, No. 2001-086330 also describes a similar technique. If an input original is judged as a special document easily in this way, a reproduction of this document can readily be prohibited. 
This technique accordingly requires a storage of reference pattern data to be applied to the copy protection for a specific document. However, it would be difficult to apply this reference pattern data which is the fixed data to the copy protection for an indefinite number of general confidential documents.
As another example, Japanese Laid-Open Unexamined Patent Application Publication, No. 07-036317 describes a technique for recognizing a confidential document dealt as a copy-prohibited document by detecting a specific mark such as “CONFIDENTIAL,” for example, printed on this confidential document indicative of its confidentiality. A print of such a specific mark indicating the document confidentiality in a confidential document is a common practice and the above-mentioned publication uses it. When an input original is judged as a special document easily in this way, a reproduction of this document can readily be prohibited. Japanese Laid-Open Unexamined Patent Application Publication, No. 07-087309 also describes a similar method.
However, when a confidential document has a print of a specific confidential mark such as “CONFIDENTIAL,” for example, a copy of this confidential document can easily be made by avoiding a print of the specific confidential mark by hiding it with a piece of paper, for example. Thus, the copy protection fails to protect the confidential document from copying.
Japanese Laid-Open Unexamined Patent Application Publication, No. 09-164739 also describes a similar technique for restricting a copying of document by embedding a watermark in an original image which is desired to be protected from copying. This technique uses a paper sheet having a specific background image to which an original image is attached. The specific background image includes a background dot pattern having a base area and a message area. From its nature, the  background dot pattern is inconspicuous in comparison with the original image and therefore it does not cause a problem when reading the original image. This technique, however, causes the background dot pattern to appear when the original image having this background dot pattern is copied. The background dot pattern of this technique is provided with a word of warning such as an “anti-copy,” for example, so that a copy of this original document is recognized easily at a glance as a confidential document which is desired to be protected from copying. Therefore, this technique has a psychological effect of restricting against the copying of document.
However, this technique produces the above-mentioned psychological effect only after the copy is made. Therefore, for the persons who do not care about the rise of the background dot pattern, the copy protection does not work at all.